Post-transcriptional gene silencing (PTGS) or co-suppression, is a common phenomenon associated with transgenes in transgenic plants. PTGS results in sequence-specific removal of the silenced transgene RNA as well as homologous endogenous gene RNA or viral RNA. It is characterized by low steady-state mRNA levels with normal (usually high) rates of nuclear transcription of transgenes being maintained. There are a number of common features or characteristics for PTGS. PTGS is                sequence-specific;        systemically transmissible;        often associated with the presence of multiple copies of transgenes or with the use of strong promoters;        frequently correlated with the presence of repetitive DNA structures, including inverted repeat T-DNA insertion patterns;        often accompanied by de novo DNA methylation in the transcribed        region, and        may be meiotically reset.        
A number of hypothetical models have been proposed to explain PTGS (see e.g. Wassenegger and Pélissier, 1998). Typically, these models suggest the involvement of a host encoded enzyme (RNA-directed RNA polymerase (RdRP)) which is proposed to use aberrant RNA as templates to synthesize small copy RNA molecules (CRNA). These cRNAs would then hybridize with the target mRNA to form duplex structures, thereby rendering the mRNA susceptible to degradation by endoribonucleases. So far, there has been no direct evidence that RdRP is involved in PTGS in plants.
An important question arising from the existing models is what type of RNA is the aberrant RNA that would be used as a template by RdRP, and in which cellular compartment RdRP would function.
Several reports have described the accumulation of unproductive or unpolyadenylated transgene RNA in plants which are post-transcriptionally silenced (Lee et al. 1997; van Eldik et al. 1998; Covey et al., 1997; van Houdt et al., 1997; Metzlaff et al.; 1997).
The following documents relate to methods and means for regulating or inhibiting gene expression in a cell.
U.S. Pat. No. 5,190,131 and EP 0 467 349 A1 describe methods and means to regulate or inhibit gene expression in a cell by incorporating into or associating with the genetic material of the cell a non-native nucleic acid sequence which is transcribed to produce an mRNA which is complementary to and capable of binding to the mRNA produced by the genetic material of that cell.
EP 0 223 399 A1 describes methods to effect useful somatic changes in plants by causing the transcription in the plant cells of negative RNA strands which are substantially complementary to a target RNA strand. The target RNA strand can be a mRNA transcript created in gene expression, a viral RNA, or other RNA present in the plant cells. The negative RNA strand is complementary to at least a portion of the target RNA strand to inhibit its activity in vivo.
EP 0 240 208 describes a method to regulate expression of genes encoded for in plant cell genomes, achieved by integration of a gene under the transcriptional control of a promoter which is functional in the host and in which the transcribed strand of DNA is complementary to the strand of DNA that is transcribed from the endogenous gene(s) one wishes to regulate.                EP 0 647 715 A1 and U.S. Pat. Nos. 5,034,323, 5,231,020 and 5,283,184 describe methods and means for producing plants exhibiting desired phenotypic traits, by selecting transgenotes that comprise a DNA segment operably linked to a promoter, wherein transcription products of the segment are substantially homologous to corresponding transcripts of endogenous genes, particularly endogenous flavonoid biosynthetic pathway genes.        
Waterhouse et al. 1998 describe that virus resistance and gene silencing in plants can be induced by simultaneous expression of sense and anti-sense-RNA. The sense and antisense RNA may be located in one transcript that has self-complementarity.
Hamilton et al. 1998 describes that a transgene with repeated DNA, i.e. inverted copies of its 5′ untranslated region, causes high frequency, posttranscriptional suppression of ACC-oxidase expression in tomato.
WO 98/53083 describes constructs and methods for enhancing the inhibition of a target gene within an organism which involve inserting into the gene silencing vector an inverted repeat sequence of all or part of a polynucleotide region within the vector.
WO 95/34688 describes methods for cytoplasmic inhibition of gene expression and provides genetic constructs for the expression of inhibitory RNA in the cytoplasm of eukaryotic cells. The inhibitory RNA may be an anti-sense or a co-suppressor RNA. The genetic constructs are capable of replicating in the cytoplasm of a eukaryotic cell and comprise a promoter region, which may be a plant virus subgenomic promoter in functional combination with the RNA encoding region.
WO95/15394 and U.S. Pat. No. 5,908,779 describe a method and construct for regulating gene expression through inhibition by nuclear antisense RNA in (mouse) cells. The construct comprises a promoter, antisense sequences, and a cis- or trans-ribozyme which generates 3′-ends independently of the polyadenylation machinery and thereby inhibits the transport of the RNA molecule to the cytoplasm.